A default UI of the control is designed so as to fit as many child C1CalendarItem controls as can fit in the current control size, taking into account the size of each single month calendar which is determined by the MonthWidth and MonthHeight properties. As a result, when control is resized, a value of the MonthCount property is changed automatically.
You can change default behavior by using custom ItemsPanelTemplate. For more details see MonthsPanel property description.